Track async causal stack traces

tests/language/vm/async_await_catch_stacktrace_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 void main() {
   x() async {
     print("Starting!");
@@ skipping 25 matching lines @@
       // #8      _Completer.completeError (dart:async/future_impl.dart:27:5)
       // #9      runAsync.async_op (this file)
       // #10     Future.Future.microtask.<anonymous closure> (dart:async/future.dart:184:26)
       // #11     _microtaskLoop (dart:async/schedule_microtask.dart:41:5)
       // #12     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
       // #13     _runPendingImmediateCallback (dart:isolate:1054:5)
       // #14     _RawReceivePortImpl._handleMessage (dart:isolate:1104:5)
 
       Expect.isFalse(stText.contains("propagateToListeners"));
       Expect.isFalse(stText.contains("_completeError"));
-      Expect.isFalse(stText.contains("main"));
     }
     print("Ending!");
   }
 
   asyncStart();
   x().then((_) => asyncEnd());
 }
 
 runAsync() async {
   throw 'oh no!';
 }